This application claims the priority of Korean Patent Application No. 2003-72056, filed on Oct. 16, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a nitride-based light emitting device and a method of manufacturing the same, and more particularly, to a nitride-based light emitting device using a p-type conductive transparent oxide thin film electrode layer and a method of manufacturing the same.
2. Description of the Related Art
The formation of an ohmic contact between a semiconductor and an electrode is of considerable importance in realizing light emitting devices such as light emitting diodes (LEDs) and laser diodes (LDs) that utilize a nitride-based compound semiconductor such as gallium nitride (GaN).
GaN-based light emitting devices are classified as top-emitting LEDs (TLEDs) and flip-chip LEDs (FCLEDs). In commonly used TLEDs, light exits through an ohmic contact layer in contact with a p-cladding layer. TLEDs need a good ohmic contact layer due to low hole concentration in the p-cladding layer. That is, TLEDs require a transparent low resistance ohmic contact layer that can provide optimal current injection to compensate for low electrical conductivity in the p-cladding layer.
TLEDs typically use a structure in which a Ni/Au ohmic contact layer is formed on a p-cladding layer. The Ni/Au layer acts as a semi-transparent ohmic contact layer having excellent specific contact resistivity of 10−4 to 10−3 Ωcm2. Annealing of the Ni/Au layer at temperature of 500 to 600° C. in an oxygen (O2) ambient leads to formation of a nickel oxide (NiO) at the interface between the p-GaN cladding layer and the Ni layer, thereby decreasing a Schottky barrier height (SBH). Thus, holes that are majority carriers can be easily injected into the surface of the p-cladding layer, thus increasing effective carrier concentration near the surface of the p-cladding layer.
Furthermore, annealing of Ni/Au on the p-cladding layer results in disassociation of a Mg—H complex in GaN, which reactivates Mg dopants by increasing the concentration on the surface of GaN. As a result of reactivation, effective carrier concentration increases above 1018 on the surface of the p-cladding layer, which causes tunneling conductance between the p-cladding layer and the ohmic contact layer containing NiO thus obtaining ohmic conductance characteristics.
However, since TLEDs using Ni/Au semi-transparent film electrode contains Au that reduce light transmittance, they suffer the limitation of realizing next generation light emitting devices with large capacity and high brightness due to their low light utilization efficiency. In a FCLED design, light is extracted through a sapphire substrate using a reflective layer in order to provide sufficient extraction of heat generated during operation while increasing light emission efficiency. However, the FCLED also suffers from problems such as high resistance due to poor adhesion and oxidation of a reflective layer.
Thus, as a solution to overcome the limitation of TELDs and FCLEDs, the use of indium tin oxide (ITO) has been proposed. ITO is a transparent conductive oxide having superior light transmittance over a semi-transparent Ni/Au used as the conventional p-ohmic contact layer. However, while increasing the output power of a light emitting device, an ITO ohmic contact layer features a high operating voltage due to a high resistance ohmic contact between p-GaN and ITO, which generates much heat. As an alternative approach, Japanese Laid-open Patent Application No. 2002-164570 discloses that high output power was obtained using p-GaN as a transparent electrode layer. However, since the above-cited invention obtains p-ZnO by codoping Ga and N, it causes many problems to actually use the p-ZnO as a transparent electrode for a p-GaN-based light emitting device. Furthermore, since it is known that the p-ZnO suffers from many reliability problems, its use as an electrode for a p-GaN light emitting device results in degradation in device reliability.